On the topic of her not showing for dinner, even as "a creature of habit" she's still 20 years old. If it's worth anything, being 24 and my little sister being 20, we tend to fall asleep at random hours of the day while we're home on our phones, watching Youtube/Netflix, cuddling with one of our cats, etc.
I have a very routine life as well, I go to work and come home at about the same time each night and I don't go partying or meet strangers online or anything like that. I don't live with my parents anymore but I do eat dinner with them frequently and we tend to eat dinner late, around 8:30 or 9 is not out of the norm. I feel like on paper, her mom not going into high alert when she didn't show sounds bad and people can't understand why she wasn't more worried. The texts we know of as I understand them were: (beginning in the afternoon) a list ingredients for baking a dessert sometime in the near future to be taken to work for Mollie's coworker on Friday 7/20, then her mother let her know she got the stuff and asked are you coming for dinner? Mollie texting at 7:30ish asking what was for dinner, her mom replying brats and then Mollie saying "ok" to those supper plans. If I had the same exhange with my mom, and she knew I was an evening jogger and she knew I had dog-sitting duties to take care of, she probably would not expect me to show until 8:30 or even 9:30 and if I did not show at all, she would assume I had fallen asleep (especially with an early morning the next day). She probably would have sent a text or called to see if I was still coming but would not have panicked, blown up my phone, or driven over to my house even though I'm like 10 min away. Normally if that happened I would wake up some time in the night, feel like a jerk and text my mom a "sorry, fell asleep" message and she would see it in the morning. She might also try my bf that night to see what was up since we live together, but as we know Dalton was out of town so she probably wasn't going to call him.
(Obviously I don't know if Mollie or her mom knew of anyone stopping by the house that night that could have changed her plans or if she often went to dinner with friends last minute but my guess is on a weeknight, probably not.)
As she heard nothing more from her that night and the shared car was still at the house the next morning, I could see my mom telling my brother well I haven't heard from her so text her to see if she is up for work and needing the car, and then my mom leaving for work and going about her day. In the moment, she would be more annoyed but I know, if she were in Mollie's mother's position now, without a doubt she would be darn near killing herself over not freaking out sooner.
So if it is worth anything, that's how I get past that piece of this story.